1. Field of the Invention
The present invention relates to an optical recording and reproducing method of a solid-state memory, in particular, a Bloch line memory.
2. Description of the Prior Art
Currently, various memory devices such as magnetic tape, winchester disks, floppy disks, optical disks, optomagnetic disks, or magnetic bubble memories are used as computer external memories, electronic file memories or still image file memories. Among such memory devices, memories excluding magnetic bubble memories involve relative movement between a recording medium such as a tape or disk and a recording/reproducing head. Therefore, along with a tendency toward high-density recording, many problems are encountered including the problem of tracking, the problem of travel and wear of the head, the problem of dust and vibrations, and, in the case of optical and optomagnetic disks, the problem of focusing.
Magnetic bubble memories do not require a mechanical drive section and have high reliability. In view of this, magnetic bubble memories are considered to be advantageous for high-density recording. However, in a magnetic bubble memory, a circular magnetic domain (bubble) formed in a magnetic garnet film having an axis of easy magnetization perpendicular to the film surface is used as 1 bit. Therefore, even if a minimum bubble (diameter: 0.3 .mu.m) attainable with the material properties of the garnet film is used, the maximum recording density per chip is several tens of megabits. Thus, unless another material such as hexaferrite or amorphous alloy is used in place of garnet, higher density cannot be obtained with a magnetic bubble memory.
As a memory capable of recording at a density higher than that attainable with a magnetic bubble memory, a Bloch line memory is receiving a lot of attention. A Bloch line memory uses, as 1 bit, a transition region wherein the direction of torsion of magnetization within the domain walls, i.e., a region (a Bloch line) is formed by a Neel domain wall structure defined by Bloch domain walls, in a magnetic domain enclosed within domain walls formed in a magnetic garnet film. As compared with a magnetic bubble memory using a circular magnetic domain (bubble) as 1 bit, a Bloch line memory allows recording at a density 100 times higher than a magnetic bubble memory using a circular magnetic domain. For example, when a garnet film having a bubble diameter of 0.5 .mu.m is used, it is possible to obtain a memory capacity of 1.6 Gbit per chip.
FIG. 1 shows a schematic structure of a conventional Bloch line memory. A substrate 1 comprises a nonmagnetic garnet, such as GGG or NbGG. A magnetic garnet film 2 is formed on the substrate 1 by LPE (Liquid Phase Epitaxy). Stripe magnetic domains 3 are formed in the film 2, and conductor line patterns 4 are formed on the film 2. A bias magnetic field HB is applied to the overall memory along the direction indicated by the arrow, i.e., downward. The data is stored in the domain walls of each stripe magnetic domain 3 in accordance with the presence or absence of a pair of Bloch lines. When the Bloch line pair is present, the data is "1", and otherwise the data is "0". The Bloch line pair is correctly present at a stable point or potential well in the stripe magnetic domain 3. When a pulse magnetic field is applied perpendicularly to the surface of the substrate, the data is sequentially transferred to the adjacent potential wells.
A conventional method of recording and reproducing data with respect to such a conventional Bloch line memory will briefly be described below.
FIG. 2 is a view for explaining a conventional method for recording in a Bloch line memory. The Bloch line memory has magnetic bubbles 5 on a major line, a conductor line 6, and stripe magnetic domains 3 as in the memory device shown in FIG. 1. Arrow A indicates the direction of current flowing to the conductor line 6. Note that a bias magnetic field (HB in FIG. 1) is applied downward from the sheet surface, and the magnetization of the stripe magnetic domains 3 and the magnetic bubbles 5 is directed in this direction. In the arrangement shown in FIG. 2, a combination of a major line as a bubble transfer path and a minor loop formed by the domain walls of the stripe magnetic domains 3 constitutes a major-minor loop as in the case of a magnetic bubble memory.
In the transfer system used in a magnetic bubble memory, e.g., in an internal rotation magnetic field system or a current drive system, the magnetic bubbles 5 are transferred onto the major line along the conductor line 6. Thus, the magnetic bubbles 5 are not arranged in front of the stripe patterns 3 for inputting a Bloch line pair but are arranged in front of the stripe patterns 3 not inputting the Bloch line pair. As a result, due to the repulsion force between a given magnetic bubble 5 and the corresponding stripe magnetic domain 3, the distance between the conductor line 6 and the stripe magnetic domain 3 is different in accordance with the presence or absence of a Bloch line pair, i.e., the presence or absence of the magnetic bubble 5. Thus, the stripe magnetic domain 3 having the magnetic bubble 5 arranged in front of it is set at a farther distance from the conductor line 6.
When a pulse current is supplied to the conductor line 6 in this state in the direction of arrow A, a local magnetic field having a direction opposite to the magnetization of the stripe magnetic domain 3 is generated near the distal end of the stripe magnetic domain 3. Then, the stripe magnetic domain 3 which is closer to the conductor line 6 contracts and a Bloch line pair is formed at the distal end of the stripe magnetic domain 3.
FIGS. 3(A) to 3(C) explain the reproducing method of the conventional Bloch line memory described above. The same reference numerals as in FIGS. 1 and 2 denote the same parts in FIGS. 3(A) to 3(C), and a detailed description thereof will be omitted. A stripe magnetic domain 3 has a Bloch line 8 at its distal end and is enclosed by domain walls 7. The arrow in the domain wall 7 indicates the direction of magnetization at the center of the domain wall, and the arrow in conductor lines 4 indicates the direction of current flowing therethrough. Referring to FIG. 3A, the stripe magnetic domain 3 is formed on a magnetic garnet film 2, and the Bloch line 8 is present in the domain wall 7. A potential well as described above is not illustrated in FIGS. 3A to 3C. Two conductor lines 4 cross the stripe magnetic domain 3. When the pulse currents of opposite directions flow as shown in FIG. 3(A), a magnetic field formed by the currents flowing in the conductor lines 4 becomes opposite to the direction of magnetization of the stripe magnetic domain 3. Therefore, the magnetic domain enclosed between the two conductor lines 4 contracts and the domain walls 7 move as indicated by dotted lines. When the currents are increased, the domain walls merge as shown in FIG. 3(B). Then, the portion of the stripe magnetic domain 3 within the merged domain walls forms a magnetic bubble 5 and is separated from the remaining portion of the stripe magnetic domain 3. After the current supply is interrupted, the Bloch line 8, similar to that formed prior to the separation of the magnetic bubble 5, is formed at the distal end of the remaining stripe magnetic domain 31, and the size of the domain is returned to the original size. FIG. 3(C) shows a case wherein the Bloch line 8 is not present. When currents are supplied to the conductor lines 4 in this state, if the Bloch line 8 is present as in FIG. 3(A), the domain walls between the two conductor lines 4 can be shifted inward and can be allowed to merge by increasing the currents. However, in FIGS. 3A and 3C wherein the Bloch line is present and not present, respectively, the directions of magnetization in the domain walls between the two conductor lines are the same in FIG. 3(A) but are opposite in FIG. 3(C). Therefore, the mutual force acting between magnetization of the domain walls upon merging become different in the two cases. That is, the current for merging the domain walls is smaller when the Bloch line 8 is present. Therefore, if the currents supplied to the conductor lines 4 are selected between the current values required for merging the domains when the Bloch line 8 is present and the current values required when the Bloch line 8 is not present, the presence or absence of the Bloch line 8 can be made to correspond to the presence or absence of the separated magnetic bubble 5. Thus, the presence or absence of the Bloch line 8 can be determined by detecting the magnetic bubble 5 by a method similar to that in the conventional magnetic bubble memory.
As described above, the conventional method of recording or reproducing a Bloch line memory utilizes bubbles in both recording and reproduction. Since this requires formation and transfer of bubbles, the overall structure becomes complex. The bit rate for recording/reproduction remains at a value corresponding to that of a magnetic bubble memory. Thus, despite of its relatively high recording density, a conventional Bloch line memory cannot provide a sufficiently high recording/reproducing speed (bit rate).